In a Long Term Evolution (LTE) system and a Long Term Evolution-Advanced (LTE-A) system, the technology of Adaptive Modulation and Coding (AMC) is applied to both downlink and uplink traffic channels to determine modulation schemes and code rates dependent upon conditions of the channels for improving spectrum efficiency of the system. Three modulation schemes, i.e., Quaternary Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16 QAM) and 64 QAM, are supported in existing LTE Release-8/9/10/11 systems, where the amounts of information transported per modulated symbol in the respective modulation schemes are 2, 4 and 6 bits respectively.
The three modulation schemes above can be supported by a Physical Downlink Shared Channel (PDSCH) among the downlink traffic channels in the LTE Rel-8/9/10 systems, and each combination of a modulation scheme and a code rate corresponds to an MCS level; and 29 MCS levels are defined in the 3rd Generation Partnership Project (3GPP) LTE Rel-11 and earlier releases as depicted in Table 1 which is a table of modulation orders and Transport Block Size (TBS) indexes of a PDSCH (simply an MCS table), where modulation orders and TBS indexes corresponding to different MCS levels in downlink scheduling are defined. In Table 1, the MCS index 0 represents a first MCS level; the MCS index 1 represents a second MCS level; and so on.
A UE is notified by the network side of a determined MCS level in 5-bit indication information in Downlink Control Information (DCI), and the UE knows a modulation scheme and a code rate used by a base station to transmit data over a PDSCH according to the MCS level indicated by the indication information received over a Physical Downlink Control Channel (PDCCH) and determines a modulation order and a TBS index corresponding to the MCS level according to the correspondence in Table 1 to determine a transport block size in the PDSCH and to demodulate and decode the data in the PDSCH.
TABLE 1MCSModulation OrderTBSIndex IMCSQmIndex ITBS0201212223234245256267278289291049114101241113412144131541416415176151861619617206182161922620236212462225623266242762528626292reserved304316
In the meantime, the UE needs to measure on the downlink channel and to feed a Channel Quality Indicator (CQI) back to the base station by transmitting a recommended modulation scheme and code rate over a Physical Uplink Shared Channel (PUSCH) or a Physical Uplink Control Channel (PUCCH); and the base station determines MCS's corresponding to the uplink/downlink channels according to the CQI fed back by the UE and a predication algorithm. The modulation schemes and the code rates are quantified, in the 3GPP LTE Rel-11 and earlier releases, into 15 CQI levels defined in the form of a 4-bit table, i.e., a CQI table as depicted in Table 2 in which modulation schemes, code rates and spectrum efficiencies corresponding to different CQI levels are defined. In Table 2, the CQI index 0 represents a first CQI level; the CQI index 1 represents a second CQI level; and so on.
TABLE 2CQIModulationSpectrumIndexSchemeCode rate × 1024Efficiency0out of range1QPSK780.15232QPSK1200.23443QPSK1930.37704QPSK3080.60165QPSK4490.87706QPSK6021.1758716QAM3781.4766816QAM4901.9141916QAM6162.40631064QAM4662.73051164QAM5673.32231264QAM6663.90231364QAM7724.52341464QAM8735.11521564QAM9485.5547
The three modulation schemes above are also supported over an uplink traffic channel PUSCH in the LTE, and 29 MCS levels are also defined in the 3GPP LTE Rel-11 and earlier releases as depicted in Table 3 which is a table of modulation, TBS indexes and redundancy version numbers of a PUSCH (simply an MCS table), where modulation orders and TBS indexes corresponding to different MCS levels in uplink scheduling are defined. A UE is notified by the network side of a determined MCS level in 5-bit indication information in DCI, so the UE knows a modulation scheme and a code rate for a PUSCH scheduled by a base station for the UE according to the MCS level indicated by the indication information received over a PDCCH and determines a modulation order and a TBS index corresponding to the MCS level according to the correspondence in Table 3 to determine a transport block size in the PUSCH and to code and modulate the data in the PUSCH.
TABLE 3RedundancyMCSModulationTBSVersion NumberIndex IMCSOrder Q′mIndex ITBSrvidx020012102220323042405250626072708280929010210011410012411013412014413015414016415017416018417019418020419021619022620023621024622025623026624027625028626029reserved1302313
The concept of a small cell has been proposed and gained significant attention along with the evolvement of the technology. The small cell has a smaller coverage area and lower transmitting power, and the rate at which the UE transmits data can be improved by deploying the small cell at a closer location to the UE (e.g., indoors, in a hotspot area, etc.).
In the small cell, there is such a shorter distance between the UE and the base station that there are typically higher uplink and downlink Signal to Noise Ratios (SNR's). A study showed that when SNR is higher (e.g., above 20 dB), using a modulation at a higher order (e.g., 256 QAM, etc.) can be benificial for further improving the throughout. In view of this, the introduction of a modulation scheme at a higher order to the LTE/LTE-A system is a feasible technical solution.
In summary, no modulation scheme at a higher order can be supported in the existing LTE/LTE-A system, thus discouraging the throughput from being further improved in an application scenario with a high SNR.